Heterojunction bipolar transistor with dielectric assisted planarized contacts and method for fabricating

ABSTRACT

A heterojunction bipolar transistor (HBT) is disclosed that includes successive emitter, base and collector and sub-collector epitaxial layers and emitter, base and collector contact metals contacting the emitter, base and sub-collector layers respectively. A passivation material is included that covers the uncovered portions of the layers and covers substantially all of the contact metals. The passivation material has a planar surface and a portion of each of the contact metals protrudes from the surface. Planar metals are included on the planar surface, each being isolated from the others and in electrical contact with a respective contact metal. A method for fabricating an HBT is also disclosed, wherein successive emitter, base, collector and sub-collector epitaxial layers are deposited on a substrate, with the substrate being adjacent to the sub-collector layer. The epitaxial layers are etched to provide locations for contact metals and emitter, base and contact metals are deposited on the emitter, base and sub-collector epitaxial layers, respectively. A self-alignment material is deposited on the surface of the substrate around the epitaxial layers and a planarization material is deposited on and covers the top surface of the HBT. The planarization material is then etched so it has a planar surface about the same level as the surface of the self-alignment material and the contact metals protrude from the planar surface. The planar metals are then deposited over the protruding portions of the contact metals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to semiconductor devices, and more particularlyto planarized contacting of semiconductor devices such as high powerheterojunction bipolar transistors (HBTs).

2. Description of the Related Art

HBTs are described in general in Wang, Introduction to SemiconductorTechnology: GaAs and Related Compounds John Wiley & Sons, 1990, pp.170-230. Some of the advantages of HBTs over other transistors such asFETs include; short transition times because of their verticalstructure, higher current handling capability per unit chip area, whichcontributes to higher output drive capabilities, and increasedtransconductance.

There is a continuing need to develop integrated circuits andcomponents, including HBTs, which operate at higher frequency or speeds,with reduced power. This design goal places a greater emphasis onincreased component integration and packing density, which results indecreased feature sizes, increased interconnection complexity and use ofnew or specialized materials.

One of the concerns in fabricating semiconductor devices such as HBTs isreliably making electrical contact to the different epitaxial layers(emitter, base and collector layer for HBTs), and isolating deviceselectrically by eliminating or rendering inactive the epitaxial layersoutside of the transistors. In fabricating conventional HBTs, epitaxiallayers are grown vertically to form the HBT's active layers. To contactthe HBT's emitter layer a contact metal can be deposited on the device'stop surface, which is generally an emitter contact layer. To contact thebase layer, the emitter contact layer and emitter layer are etched(except under the-emitter contact) to provide a surface on the baselayer for depositing a contact metal. To contact the collector layer,portions of the emitter contact layer, emitter layer, base layer andcollector layers are etched to provide a surface on a sub-collectorlayer for depositing the contact metal.

Before electrical connection to the contact metals, conventional HBTsare covered with a passivation material. The emitter, base and collectormetals are then contacted through the passivation material, by etching avia pathway through the passivation material down to each contact metal.A conductive material is then deposited in the etched areas to formconductive vias to the contact metals. Electrical connection is made tothe HBT's active layers by connection to the conductive vias.

One disadvantage of this technique is that it cannot be used to reliablycontact high speed HBTs. To increase the speed of HBTs, the emitterbecomes smaller and the size of the emitter correlates to the size ofits contact metal. As the emitter is reduced, a point is reached wherethe emitter metal cannot be reliably contacted by using conductive vias.The resolution and alignment limits of lithography and etch systemslimit how small the device features can be for HBTs. The smallestresolution for etching a via though polymide or BCB over an emittermetal is approximately 0.5 μm. The emitter should be 0.2 μm wider thanthe width of the via to provide a margin of error in case the via is notperfectly aligned over the emitter metal. Accordingly, the smallest theemitter can be is approximately 0.75 μm wide.

Another concern is that for smaller emitters the via etch may not alignwith the emitter metal. This can result in the etch extending beyond theemitter metal to the epitaxial layer. When the via etch is filled withthe conductive material to form the via, the conductive material canform a short to the epitaxial material that bypasses the emitter metal,emitter contact layer and emitter layer. This naturally results in agreater number of fabrication errors, making the HBTs less reproducible.

Planarization has been used to remedy surface topologies that can createproblems for a semiconductor device's performance and survivability [SeeU.S. Pat. No. 4,996,165 to Chang et al.]. Variations in feature height,topography or morphology can lead to stress in subsequently depositedlayers or materials and height variations in one layer can make precisecontrol of the dimensions of subsequent layers difficult. The featuresof a semiconductor device are formed by depositing a layer ofphotoresist on an upper layer of the structure and developing it in adesired pattern. After development and etching, some photoresist remainson the upper surfaces of the features. A layer of dielectric materialsuch as SiO is deposited across the semiconductor structure to a depthsubstantially the same as the height of the tallest features. Theremaining photoresist is then removed along with the dielectricdeposited thereon. A layer of polymide is deposited on the upper surfaceof the SiO and features, and extends into depressions between to controlthe height variations.

SUMMARY OF THE INVENTION

The present invention seeks to provide a semiconductor device such as anHBT, where the size of the device is not limited by the resolution ofvia etching. The invention also seeks to provide a reliablesemiconductor device that is readily reproducible.

These goals are realized by a multi-layered semiconductor embodiment ofthe present invention, which includes a plurality of successiveepitaxial layers and a plurality of contact metals electricallycontacting a respective one of the epitaxial layers. A planarizationmaterial is included that covers the uncovered portions of the epitaxiallayers and covers substantially all of the contact metals. Theplanarization material has a substantially planar surface from which aportion of each of the contact metals protrudes. Planar metals areincluded on the planar surface, with each of the planar metalselectrically isolated from the other and in electrical contact with theprotruding section of a respective one of the contact metals.

The present invention is particularly applicable to HBTs that includesuccessive emitter, base and collector layers and emitter, base andcollector contact metals electrically contacting their respective layer.A passivation material is included that covers the uncovered portions ofthe emitter, base and collector layers and covers substantially all ofthe contact metals. The passivation material has a substantially planarsurface from which a portion of each of the contact metals protrudes.The HBT also includes a first, second and third planar metal on thesubstantially planar surface, each planar metal in electrical contactwith a respective contact metal.

The present invention also discloses a method for fabricatingmulti-layered semiconductor devices, one embodiment of which includesdepositing a plurality of successive epitaxial layers on a substrate,with the substrate extending laterally beyond said epitaxial layers. Aplurality of contact metals are deposited on the epitaxial layers, and alayer of self alignment material is deposited on the substrate, aroundbut not contacting the epitaxial layers or contact metals. Aplanarization material is deposited over the self-alignment material,the epitaxial layers and the plurality of contact metals. Theplanarization material is then etched so it has a planar surface aboutthe same level as the surface of the self-alignment material, and theplurality of contact metals protrude from the planar surface. Planarmetals are then deposited on the planar surface with each of the planarmetals isolated from the others and each in electrical contact with arespective one of the contact metals.

Another embodiment of a method according to the present invention isalso disclosed that is particularly adapted to fabricating HBTs.Successive emitter, base, collector and sub-collector epitaxial layersare deposited on a substrate, with the substrate being adjacent to thesub-collector layer. The epitaxial layers are etched to providelocations for metal contacts, and emitter, base and contact metals aredeposited on the emitter, base and sub-collector epitaxial layers,respectively. A self-alignment material is deposited on the surface ofthe substrate around the epitaxial layers and a planarization materialis deposited on and covers the top surface of the HBT. The planarizationmaterial is then etched so it has a planar surface about the same levelas the surface of the self-alignment material and the contact metalsprotrude from said planar surface.

The new device and method allows for fabricating devices without relyingon etching conductive vias for contacting the contact metals. The newmethod results in devices that are more reproducible and reliable andless complicated to fabricate. Also, the size of the device is notlimited by the resolution of the via etching. Smaller devices can befabricated, which can result in faster devices that consumes less power.

These and other further features and advantages of the invention will beapparent to those skilled in the art from the following detaileddescription, taken together with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of one embodiment of a multi layeredsemiconductor device (HBT) according to the present invention;

FIG. 2 is a flow diagram for one embodiment of a fabrication methodaccording to the present invention;

FIG. 3 is a flow diagram of another embodiment of a fabrication methodaccording to the present invention, which is particularly adapted toHBTs;

FIG. 4 is a sectional view of an embodiment of an HBT according to thepresent invention in one of its initial fabrication steps using themethod of FIG. 3;

FIG. 5 is sectional view of the HBT in FIG. 4 after a subsequentfabrication step using the method of FIG. 3;

FIG. 6 is a sectional view of the HBT in FIG. 5 after a subsequentfabrication step using the method of FIG. 3;

FIG. 7 is a sectional view of the HBT in FIG. 6 after a subsequentfabrication step using the method of FIG. 3;

FIG. 8 is a sectional view of the HBT in FIG. 7 after a subsequentfabrication step using the method of FIG. 3;

FIG. 9 is a sectional view of the HBT in FIG. 8 after a subsequentfabrication step using the method of FIG. 3; and

FIG. 10 is a sectional view of the HBT in FIG. 9 after a subsequentfabrication step using the method of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows one embodiment of an HBT 10 according to the presentinvention. It is formed on an electrically semi-insulating InP substrate12, which supports an InP sub-collector 14, with the substrate extendinglaterally beyond the sub-collector layer 14. The sub-collector 14 isheavily doped n+ so that it is substantially conductive. Thesub-collector 14 is typically about 0.5-1 micron thick, with a dopantconcentration of greater than 1×10¹⁹/cm³. The purpose of thesub-collector 14 is to establish an electrical contact with thecollector layer 16, which normally directly contacts the upper surfaceof the sub-collector 14. The collector 16 is typically about 0.2-1.0microns thick, with a dopant concentration of about 1×10¹⁶ to1×10¹⁷/cm³. The collector layer is typically doped with silicon (Si)during the epitaxial growth.

A heavily doped InGaAs base layer 18 is included on the collector layer16 opposite the sub-collector layer 12, with the base layer 18 coveringmost of the collector layer 16. The sub-collector 14 layer extendslaterally beyond said collector layer 16. The base layer 18 is typicallyabout 300-1000 Angstroms thick and carbon doped p++ to a concentrationof about 5×10¹⁹/cm³. An InP emitter layer 20 is on the base layer 18 andnormally directly contacts the base layer's upper central surface. Theemitter layer 20 is typically about 400 to 2000 Angstroms thick and ndoped to a concentration of about 3×10¹⁷/cm³. The emitter couldalternatively be formed of InAlAs. The emitter layer 20 has an InGaAsemitter contact layer 22, which is typically n doped to a concentrationof about 1×10¹⁹/cm³ and is greater than approximately 200 Angstromsthick. Emitter, base and collector contact metals 24, 26, 28 areincluded to electrically contact the emitter, base and sub-collectorlayers 20, 18, 16, respectively. The emitter contact metal 24 is on andcovers the emitter contact layer 22. The base contact metal 26 isdeposited directly on the base layer 18 where the emitter layer 20 andemitter contact layer 22 have been etched. The collector contact metal28 is deposited on the sub-collector layer 12 in an area where theemitter contact layer 22, emitter layer 20 and base layer 18 have beenetched.

A layer of SiO 30 surrounds, but does not cover or contact the activelayers or contact metals of the HBT 10. The SiO layer 30 allows forself-alignment during fabrication, so various device features can bedefined with the same photoresist pattern. This avoids the relativelywide spaces that would otherwise be needed for photolithographyalignment tolerances. Using self-alignment, the spacing between regionsdefined on different mask layers can be reduced.

In accordance with the present invention, electrical contact is not madeto the contact metals 24, 26 and 28 through conductive vias. Instead,contact is made by depositing metals on a planarized surface inelectrical contact with the HBT's contact metals. A planarizationmaterial 32, such as a polymide or Benzocyclobutenes (BCB), which alsopassivates the surfaces of the active layers, fills the open spaces ofthe HBT 10 between the SiO layer, covering the HBT's active layers andmost of the contact metals 24, 26, 28. Only a small section of the topof the metals 24, 26, 26 is not covered by the passivation material 32.The top surface of the passivation material 32 should be atsubstantially the same level as the top surface of the SiO layer 30, butcan be slightly lower (as shown) or slightly higher.

The top surface of the passivation material and the SiO layer provide asubstantially level surface 33 for electrically contacting the contactmetals 24, 26, 28. Electrical contact is made to the emitter metal 24 bydepositing a first planar metal 34 over the portion of the emitter metal24 that protrudes from the passivation material 32. Electrical contactis similarly made to the base metal 26 by depositing a second planarmetal 36 base metal's protruding portion. The second planar metal 36takes advantage of the adjacent surface of the SiO layer to provide alarger planar surface. Electric contact is made to the collector metal28 by depositing a third planar metal 38 over the protruding portion ofthe collector metal 28 and third planar metal 38 also takes advantage ofthe adjacent surface of the SiO layer 30. Each of the planar metals istypically 1.0 μm thick and can be made of any conductive material, butis preferably made of a metal such as Ti, Pt, or Au. All three planarmetals can be deposited at the same time.

By electrically contacting the contact metals 24, 26, 28 over a planarsurface instead of through conductive vias, the size of the emitter isnot limited by the tolerance of the via lithography or etching process.Electrical contact is made without having to etch for a via. The size ofthe planar metals 34, 36, 38 can remain the same independent of the sizeof the emitter and its contact metal. Using smaller emitters, HBTssmaller can be fabricated with smaller features that operate at anincreased speed with reduced power consumption.

FIG. 2 is a flow diagram of one embodiment of a semiconductorfabrication method 40 according to the present invention. The method 40can be used to fabricate many different semiconductor devices and isparticularly adapted to providing planar metals to electrically contactthe device's epitaxial layers.

In step 42, the epitaxial layers are deposited on a substrate usingstandard deposition techniques. In step 44, metal contacts are depositedon the epitaxial layers using standard deposition techniques such assputtering. In step 46, a self-alignment layer is deposited on thesubstrate around the epitaxial layers. The top surface of theself-alignment layer is preferably just below the top of the contactmetals. In step 48, a planarization material is then deposited, whichcovers the exposed surface of the entire device.

In step 50, the planarization material is etched using standard etchingtechniques such as reactive ion etching (RIE), so that the material issubstantially at the same level as the surface of the self-alignmentlayer and a portion of each of the contact metals protrudes from thesurface of the planarization material. In step 52, respective planarmetals are deposited over protruding portions of the contact metal,again using standard deposition techniques. The planar metals are on thesurface of the planarization material or on the surface of theplanarization material and the self-alignment layer. Each of the planarmetals is preferably electrically isolated from the other(s) andcontacts a respective contact metal.

FIG. 3 is a flow diagram of another embodiment of a fabrication method60 according to the present invention, that is particularly adapted tofabricating HBTs of the type shown in FIG. 1. FIG. 3 will be referencedin conjunction with FIGS. 4-10, which show the HBT 10 at different stepsin the fabrication method 60 of FIG. 3. Where the layers/features arethe same as those in FIG. 1, the same reference numerals will be used inFIGS. 4-10.

In step 62 and as shown in FIG.4, the active layers 64 of an HBT aredeposited on a substrate 12 by epitaxial growth techniques such asmolecular beam epitaxy (MBE) or metal-organic chemical vapor phasedeposition (MOCVD). The active layers 64 generally comprise thecollector, base and emitter layer 16, 18, 20, with an emitter contactlayer 22 being included on the emitter layer 20. The active layers 64can be made from many different material systems including but notlimited to the InP/InGaAs material system described above in FIG. 1. Asub-collector layer 14 is included adjacent the collector layer 16, withall of the layers being fabricated on a substrate 12. The sub-collector14 is preferably adjacent to the substrate 12 and both can be made ofmany different materials, with suitable materials being InP or InGaAs.

In step 66 and as further shown in FIG. 4, an emitter metal 24 isdeposited on the HBT 10 using standard deposition techniques, such assputtering. In a preferred method the emitter metal 24 is deposited onthe emitter contact layer 22. In step 68 as shown in FIG. 5, the emittercontact layer 22 and the emitter layer 20 are etched to the base layer18, except for the portion of these layers under the emitter contactmetal 24, with a suitable etching technique being RIE. In step 70 asshown in FIG. 6, the base contact metal 26 is then deposited directly onthe base layer 18, with a suitable deposition technique also beingevaporation.

In step 72 as shown in FIG. 7, the base layer 18 and collector layer 16are etched to the sub-collector layer 14, except for the portions ofthese layers under the base and emitter contact metals 24 and 26 and thearea between them. In 74 as also shown in FIG. 7, the collector contactmetal 28 is deposited on the sub-collector layer 14, also with asuitable method being evaporation.

In step 76 as shown in FIG. 8, the SiO layer 30 is deposited around theHBT's active layers 64 and contact metals. This can be accomplished byfirst depositing a photoresist material (not shown) over the activelayers 64 and contact metals 24, 26, 28. The photoresist is then etchedaround the active layers 64 and contacts 24, 26, 28, down to thesubstrate. The SiO layer 30 can then be deposited and the photoresistmaterial can be lifted off to again reveal the active layers and contactmetals.

In step 78 as shown in FIG. 9, a planarization material 32 is thendeposited over the entire HBT, including the SiO layer 30. The preferredplanarization material also passivates the epitaxial layers and asdescribed above, a suitable material can be a polymide orBenzocyclobutenes (BCB) or any material that passivates the surfaces ofthe epitaxial layers and also can be etched to a planar surface.

In accordance with the present invention, in step 80 as shown in FIG.10, the passivation material 32 is etched to reveal the top portions ofthe contact metals and to provide a substantially level surface 33 (seeFIG. 1) with the surface of the SiO layer 30. In step 82 as shown inFIG. 1, first, second and third planar metals 34, 36 and 38 aredeposited over their respective contact metals 24, 26 and 28 forelectrical contact to the underlying HBT.

Although the present invention has been described in considerable detailwith reference to certain preferred configurations thereof, otherversions are possible. As described above, the invention can be usedwith many different semiconductor devices beyond HBTs. Also, differentmethods can be used to fabricate devices with planar contacts accordingto the present invention. In the methods shown, fewer or additionalsteps can be used and the steps can take place in different sequences.Therefore, the spirit and scope of the appended claims should not belimited to the preferred versions in the specification.

1. A heterojunction bipolar transistor (HBT), comprising: successiveemitter, base and collector layers; emitter, base and collector contactmetals electrically contacting the emitter, base and collector layers,respectively a passivation material covering the uncovered portions ofsaid emitter, base and collector layer and covering substantially all ofsaid contact metals, said passivation material having a substantiallyplanar surface, the uncovered portions of said contact metals protrudingfrom said passivation material above said substantially planar surface;and first, second and third planar metals electrically contacting arespective one of said contact metals, each of said planar metals onsaid substantially planar surface, and covering said protruding portionof its respective contact metal.
 2. The HBT of claim 1, furthercomprising a sub-collector layer adjacent to and extending laterallybeyond the edge of said collector layer, said collector contact metaldeposited on said sub-collector layer.
 3. The HBT of claim 2, furthercomprising a substrate adjacent to and extending laterally beyond saidsub-collector layer.
 4. The HBT of claim 3, further comprising aself-alignment layer on the same surface of said substrate as saidsub-collector, said self-alignment layer surrounding but not contactingsaid emitter, base, collector and sub-collector layers or said contactmetals, the surface of said self alignment layer forming a substantiallyplanar surface.
 5. The HBT of claim 1, wherein said passivation materialpassivates the surfaces of said emitter, base and collector layers. 6.The HBT of claim 1, further comprising an emitter contact layersandwiched between said emitter layer and said emitter contact metal. 7.The HBT of claim 1, wherein said emitter layer comprises InP.
 8. The HBTof claim 1, wherein said base layer comprises InGaAs.
 9. The HBT ofclaim 1, wherein said collector layer comprises InP or InGaAs.
 10. TheHBT of claim 2, wherein said sub-collector layer comprises InP orInGaAs.
 11. The HBT of claim 3, wherein said substrate comprises InP.12. The HBT of claim 4, wherein said self-alignment layer comprises SiO.13. A heterojunction bipolar transistor (HBT), comprising: successiveemitter, base, and collector layers; a sub-collector layer adjacent tosaid collector layer opposite said base layer and extending laterallybeyond said collector layer; a substrate adjacent to said sub-collectorlayer opposite said collector layer and extending laterally beyond saidsub-collector layer; emitter and base contact metals on said emitter andbase layers, respectively, said emitter and base contact metalsextending from their corresponding layers; a collector contact metal onsaid sub-collector layer, said collector contact metal extending fromsaid sub-collector layer; a self-alignment layer on said substratesurrounding but not contacting said emitter, base, collector orsub-collector layers or said metal contacts; a planarization materialfilling the area between said self-alignment layer and said emitter,base, collector and sub-collector layers and substantially all of saidcontact metals.
 14. The HBT of claim 13 wherein said planarizationmaterial has a planar surface from which a portion of said contactmetals protrudes.
 15. The HBT of claim 14, wherein said planar surfaceof said planarization material and the surface of self-alignment layerform a substantially level surface.
 16. The HBT of claim 15, furthercomprising a first, second and third planar metal, each on said levelsurface and each electrically isolated from the other and in contactwith a respective one of said emitter, base and collector metals. 17.The HBT of claim 13, wherein said emitter, base, collector andsub-collector layers, and said substrate are made of a material from thegroup consisting of InP and InGaAs.
 18. A semiconductor device,comprising: a plurality of successive epitaxial layers, a plurality ofcontact metals electrically contacting a respective one of saidepitaxial layers; a planarization material covering the uncoveredportions of said plurality of epitaxial layers and coveringsubstantially all of said plurality of contact metals, saidplanarization material having a substantially planar surface, a portionof each of said plurality of contact metals protruding from saidplanarization material above said substantially planar surface, and aplurality of planar metals on said substantially planar surface, each ofsaid plurality of planar metals covering and in electrical contact witha respective one of said plurality of contact metals.
 19. The device ofclaim 18, further comprising a substrate, said epitaxial layers on asurface of said substrate and said substrate extending laterally beyondsaid plurality of epitaxial layers.
 20. The device of claim 19, furthercomprising a self-alignment layer on the same surface of said substrateas said plurality of epitaxial layers, said self-alignment layersurrounding but not contacting said plurality of epitaxial layers orsaid contact metals, the surface of said self alignment layer and saidplanar surface forming a substantially level surface.
 21. The device ofclaim 18, wherein said planarization material passivates the surfaces ofsaid epitaxial layers.
 22. The device of claim 18, wherein saidplurality of epitaxial layers and said substrate are made of a materialfrom the group consisting of InP and InGaAs.
 23. The HBT of claim 13,further comprising an emitter contact layer on said emitter layeropposite said base layer, said emitter contact metal on said emittercontact layer.
 24. The HBT of claim 1, wherein said emitter, base, andcollector contact metals are positioned substantially on theirrespective layer.
 25. The HBT of claim 1, wherein at least one of saidemitter, base, and collector contact metals extending up from itsrespective layer.